using System;

using System.Collections.Generic;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

      string[] partNumbers= { "172.16.15.1", 

                              "172.16.15.1/16", 

                              "172.16.15.255",

                              "172.16.15.255/16",

                              "172.236.15.16/3222"

                            };

      //before:     

      //string pattern = @"([1-9]|[1-9]\d|1\d{2}|2[0-4]\d|25[0-5])(\.(\d|[1-9]\d|1\d{2}|2[0-4]\d|25[0-5])){3}\/\d+";

    　//after

      string pattern = @"^([1-9]|[1-9]\d|1\d{2}|2[0-4]\d|25[0-5])(\.(\d|[1-9]\d|1\d{2}|2[0-4]\d|25[0-5])){3}\/(\d|[1]\d|[2]\d|[3][0-2])$";

      //主要就是([1-9]|[1-9]\d|1\d{2}|2[0-4]\d|25[0-5])這個pattern, 裡面包含了ip token可能會有的幾種狀況

      //ex: 只有1位數, 2位數, 1xx, 2xx, 25x等

      //    

      //後面三個class規則類似，差別僅在需要以.開頭

      foreach (string partNumber in partNumbers)

         Console.WriteLine("{0} {1} a valid ip address.", 

                           partNumber, 

                           Regex.IsMatch(partNumber, pattern, RegexOptions.IgnoreCase) 

                                         ? "is" : "is not");

    }

}